Spin-polarization effect on electron attachment to atomic hydrogen on liquid helium surface

No Heading Reaction rates of electron attachment to atomic hydrogen are measured as a function of magnetic field. The reaction takes place in a two-dimensional mixture of hydrogen atoms and electrons on liquid helium surface. Surface electron density, measured by using vibrating capacitor electrometer technique, decreases when H atoms are introduced. Applied high magnetic field suppresses electron attachment, H + e^– H^–, as well as hydrogen recombination, H + H H₂. Since the electronic state of negative hydrogen, H^–, is spin singlet, electron attachment is suppressed by spin-polarization. Possible microscopic mechanisms to explain the measured magnetic field dependence of the reaction kinetics are discussed.PACS numbers: 67.65.+z, 68.     

ESR on adsorbed doubly spin-polarized atomic hydrogen

Studies of the ESR spectrum of spin-polarized atomic hydrogen adsorbed on a liquid helium film are presented. The absorption peaks associated with the surface atoms are displaced relative to that of the gaseous atoms due to the electronic dipole-dipole interaction, which does not average to zero for atoms moving in two dimensions. This phenomenon, first observed by Reynolds et al., allows the surface atom density to be measured directly and, through the lineshape, information on the dynamics of the 2-D gas can in principle be obtained. Here we present a more detailed study, with a better characterized substrate for the helium film.Using only the assumption that the ideal gas approximation is valid for the experimental conditions, we find that the binding energy of hydrogen atoms to the liquid helium surface is 1.03 (2) K. Although these measurements of the binding energy are not the most accurate, they are the most direct.The ESR lineshape of the absorption peak of the bulk atoms is determined by the inhomogeneity of the applied magnetic fields, whereas the resonance lineshape of the adsorbed atoms, which is very asymmetric and much broader than the main resonance, is clearly due to some other mechanism. In spite of a considerable effort to explain the observed lineshapes, we have not reached satisfactory conclusions.     

The surface tension meniscus of liquid helium

A technique is described for calculating the shape of the surface tension meniscus at a vertical wall, in the presence of Van der Waals forces. The method of analytic continuation is used to compute a solution of the relevant differential equation. Results (believed accurate to better than 0.5%) are presented for He³ and for He⁴ at a variety of film flow rates. It is found that the shape of the classical meniscus remains unaltered over most of its length, but it is displaced horizontally relative to the wall by a flow dependent amount. The results are contrasted with those from a previous calculation.     

The torque on a rotating disk in the surface of a liquid with an adsorbed film

Summary In this paper we consider the problem of calculating the resistive torque on a disk rotating slowly with constant angular speed in the surface of a liquid with an adsorbed surface film. Using the method of complementary representations for generalised axially symmetric potential functions, the boundary-value problem for the azimuthal velocity component is reduced to the solution of a Fredholm integral equation of the second kind. This equation is solved numerically and asymptotically for all values of the ratio of the surface shear viscosity of the film to the viscosity of the substrate fluid, and values calculated for the substrate and film torques on the disk. The results are compared with previous work of Goodrich and his co-workers.     

火箭推进剂液氢中氦的分析方法

使用气相色谱法分析氢中氦时,在常用色谱柱上氦出峰在氢后面,并且位置很近,较大的氢峰会掩盖氦峰。针对上述问题,研制了一种新的色谱柱固定相,用于色谱法分析氢中氦组分。给出了这种固定相对氢、氦组分的分离原理、制作方法和实验过程。实验结果表明,采用气相色谱热导检测器,配用金属钯做色谱柱固定相,可成功地完成对氢、氦组分的分离和检测。     

Electronic surface states on liquid helium

Beginning with a microscopically obtained density profile near the free surface of liquid helium an analysis of the electronic surface states, both inside and outside, is carried out. For the potential barrier seen by electron bubbles trapped inside the surface layer we find an upper bound of 38 K, in comparison with experimental values ranging from 25 to 30 K. For outside states we find a binding energy of 9 K and a hydrogenlike spectrum. There are no adjustable parameters in the theory.     

Surface state electrons on a hydrogen film. 2. Influence of adsorbed helium films

We have investigated the conductivity of surface state electrons on liquid⁴He films supported by a quench-condensed thin solid hydrogen substrate. A pronounced dip in the ac-conductivity was observed as the He film thickness reached about 10 layers. This phenomenon was studied at different temperatures (range between 1.6 and 2 K), electron densities from 0.9 to 19×10⁸ cm^–2, frequencies between 1 and 100 kHz, and for various amplitudes of the ac driving voltage. Ripplon scattering and a ripplonic polaron are discussed as sources for the conductivity dip.     

Alkali dimers on the surface of liquid helium

A recent paper by Ancilottoet al. (Zeitschrift für Physik B, in press), presented calculations of adsorption energies and the geometry of a surface dimple for alkali atoms bound to the surfaces of quantum liquids (⁴He,³He, H₂). Here we present a study of the adsorption of two alkali dimers (Li₂, Na₂) on the surface of liquid helium. The calculations employ a model of an abrupt interface formulated by Ancilotto et al. as well as one using a diffuse interface. Our conclusion its that the dimers are bound to the surface more strongly than their respective monomers. In the case of dimers there is an additional degree of freedom-the orientation of the molecular axis relative to the surface. We study the influence of molecular anisotropy on adsorption by comparing the cases of erect and spinning flat orientations and conclude that the latter is energetically favored.     

Electron-ripplon scattering on the surface of liquid helium

We use an adiabatic development of the electron wave function to calculate the scattering matrix elements for the electron-ripplon interaction. In this treatment the nonadiabatic terms in the total Schrödinger equation are responsible for the scattering. Our calculation shows that the magnitude of the scattering matrix element for this interaction depends critically on the magnitude of the electron wave function at the surface.     

Conductivity of electrons on a liquid helium surface in high magnetic fields

The elementary processes of electron scattering on a liquid helium surface in the limit of a high magnetic field are considered. For the scattering on helium vapor atoms, the adiabatic approach is developed, which takes into account the slow thermal motion of atoms. The scattering on ripplons is considered in a lowest, nonvanishing order of the perturbation theory.     

Interfacial friction of submonolayer helium films adsorbed on the surface of graphite

The frictional coupling between an adsorbed helium film and the surface of graphite has been studied using torsional oscillator methods and exfoliated graphite as a substrate, in the temperature range 50mK to 2K. The observed period shift is consistent with the film decoupling from the substrate as the temperature is reduced, and is accompanied by a maximum in dissipation. At coverages below registry and in the incommensurate solid region the data are well described by Debye relations with a thermally activated relaxation time. This would correspond to a vanishing of the interfacial friction for the solid monolayer as T - 0. At coverages between 0.064Ų and 0.08Å-² a double peaked structure in the dissipation is observed. The results relate to the work of Krim et al.¹ who have used a quartz microbalance to study the nanotribology of adsorbed monolayers. In the present work the use of the graphite substrate offers the possibility of studying these effects on a system with a rich and well characterised submonolayer phase diagram.     

液氢温区冷氦增压系统试验研究

针对某型号火箭冷氦增压系统,建设了液氢温区冷氦增压系统试验平台,通过试验得到了冷氦气源压力和温度、冷氦加热器性能、模拟贮箱压力和温度的变化规律.结果表明:低温制冷机组配合高压低温换热贮罐可以真实模拟箭上的冷氦气源;根据对增压过程中贮箱压力的分析表明排气方式可以真实模拟箭上推进剂消耗过程中贮箱压力的变化情况;另外,试验中压力信号器、电磁阀和贮箱工作正常,验证了火箭冷氦增压系统的可靠性.     

Interaction of a dispersed liquid flow with a heated surface

A study is made of certain features of the interaction of a flow of dispersed liquid with a heated surface in the dry regime and under conditions attending a transition to the wet regime.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 40, No. 1, pp. 35–40, January, 1981.     

氦气作为增压气体排出贮罐内液氢过程的CFD分析

基于计算流体力学方法,数值模拟了用常温氦气作为增压气体压出贮罐内液氢过程的流动和传热传质特性。构建了基于二维轴对称的VOF多相流以及包含氢气和氦气组分流动的气相多组分数值模型,液氢界面相变传质基于Hertz-Knudsen方程计算。分析了排出过程贮罐内压力、温度、液位及液氢相变率随时间的变化,重点考察气相出现在贮罐出口时间,以及此时气相中氦气含量。发现刚开始增压时,高温氦气和低温氢气传热只发生在氦气进口附近,贮罐内压力增加较慢,液氢界面不存在蒸发现象。随着进入氦气增加,贮罐内气相温度逐渐形成分层,在一定时刻,液面上气体温度开始上升,触发沸腾蒸发,导致压力快速增加。由于贮罐出口液体外流导致的减压效应远小于气相空间的压力增速,贮罐压力急剧增加并超过氦气入口,部分低温气体混合物从入口倒流出贮罐,同时使氦气入口处温度降低。由于贮罐内压力增加,底部液氢出口流量随时间呈线性增加。计算结果揭示了液氢贮罐增压流出过程复杂的流动和传热传质特性,对低温液体的储运有实际工程指导意义。     

Analysis of data from spilling experiments performed with liquid hydrogen

This work describes the modelling of liquid hydrogen release experiments using the ADREA-HF 3-D time dependent finite volume code for cloud dispersion, jointly developed by DEMOKRITOS and JRC-Ispra. The experiments were performed by Batelle Ingenieurtechnik for BAM (Bundesanstalt fur Materialforschung und Prufung), Berlin, in the frame of the Euro-Quebec-Hydro-Hydrogen-Pilot-Project and they mainly deal with LH2 near ground releases between buildings. In the present study, the experimental trial #5 was assumed for simulation due to the fact that in this release the largest number of sensor readings were obtained. The simulations illustrated the complex behaviour of LH2 dispersion in presence of buildings, characterized by complicated wind patterns, plume back flow near the source, dense gas behaviour at near range and significant buoyant behaviour at the far range. The simulations showed the strong effect of ground heating in the LH2 dispersion. The model also revealed major features of the dispersion that had to do with the "dense" behaviour of the cold hydrogen and the buoyant behaviour of the "warming-up" gas as well as the interaction of the building and the release wake. Such a behaviour was in qualitative and even quantitative agreement with the experiment. The results are given in terms of concentration time series, scatter plots, contour plots, wind field vector plots and 3-D concentration wireframes. Given all experiment uncertainties, the model gives reasonable results on concentrations levels.     

A reciprocating liquid helium pump used for forced flow of supercritical helium

The performance of a small double acting piston pump for circulating helium in a closed heat transfer loop is described. The pump was manufactured by LINDE AG, Munich, West Germany. The measured flow rate of supercritical helium was about 17 gs^?1 (500 lhr^?1) with a differential pressure of Δρ = 0.5 × 10⁵Nm^?2 at a working pressure of p = 6 × 10⁵Nm^?2. At differential pressures beyond 0.5 × 10⁵Nm^?2 the volumetric efficiency decreases.     

Dynamic measurement of the surface tension of liquid helium with a two-dimensional electron probe

Electrons deposited above the free surface of liquid helium exhibit coupled electron-ripplon modes whose frequencies are simply related to the surface tension of helium. The hydrodynamic expression for the ripplon dispersion relation for pure⁴He is shown to be valid up to the wavevector range 10⁵ cm^–1<q<2×10⁶ cm^–1 of the coupled modes. For a small amount of³He (X ₃10^–6–10^–8) added to pure⁴He, we measure the variation of the surface tension in the 60–600 mK temperature range. These experiments, which cover the frequency range v10–300 MHz, agree fairly well with previous static (capillary) or low-frequency (surface sound velocity) experiments.     

Escape rate of two-dimensional electrons on a liquid helium surface around 1 K

Electron escape from a two-dimensional surface state on liquid helium to three-dimensional free space is studied. A thermal-activation-type temperature dependence of the escape rate is observed at 1.1>T>0.9 K for the first time. The mechanism of electron escape is discussed.